The present invention relates to a switching power supply which switches current flowing through an inductor by using a switching element thereby to obtain a boosted DC output.
A tuner provided in a television etc. employs an arrangement wherein a tuning frequency is changed by changing a tuning voltage. The tuning voltage is generally a voltage slightly exceeding 30 volt at the maximum. Thus, the tuning voltage cannot be generated from the operation power supply in a vehicle-mounted television having the operation power supply of 12 volt. Accordingly, in this case, a booster circuit is provided in order to obtain the DC voltage slightly exceeding 30 volt used for generating the tuning voltage. FIG. 2 shows the prior art of the booster circuit which is generally configured by the known arrangement of a chopper type booster circuit. That is, a transistor Q7 performing the switching operation in accordance with the output of an oscillation circuit 53 is provided on the output side of an inductor L7. The oscillation of the oscillation circuit 53 is controlled by the output of an error detection circuit 54 for detecting error voltage of the DC output, whereby the voltage of the DC output is stabilized (this technique is hereinafter called a the first prior art).
One of the prior arts for generating the tuning voltage is proposed in the Unexamined Japanese Utility Model No. Sho 62-178619. FIG. 3 is a block diagram showing this technique. This technique will be explained with reference to the figure. In this technique, a positive power supply +B is introduced to a zener diode D7 through a serial resistor R21 for voltage drop, thereby obtaining DC output necessary for generating the tuning voltage from the cathode of the zener diode D7. A tuning voltage limiting circuit 51 for limiting the voltage to a predetermined voltage is provided between the cathode of the zener diode D7 and a PWM circuit 52 for generating the tuning voltage. When receiving a particular band, the voltage of the DC output supplied to the PWM circuit 52 is limited to the predetermined voltage, whilst when receiving other bands, the voltage supplied to the PWM circuit 52 is made equal to the cathode voltage of the zener diode D7 (this technique is hereinafter called as the second prior art).
However, according to the first prior art, since the error detection circuit 54 for detecting the error voltage of the DC output is required in order to stabilize the voltage of the DC output, there arises a problem that the circuit configuration becomes complicated. According to the second prior art, the serial resistor R21 and the zener diode D7 are employed in order to stabilize the voltage supplied to the tuning voltage limiting circuit 51. Thus, since the two elements are required for stabilizing the voltage, the number of the elements is increased. In this case, since the serial resistor R21 serves as an element for causing the voltage drop for absorbing the voltage variation, there arises a problem that the serial resistor R21 can not be eliminated.